I want to calculate the falling velocity of my drop in 3D VOF model. So far, I've created an iso-surface of volume frac. on the drop, and use surface integral panel to calculate the area-weighted average of Y velocity (the vertical coordinate) over the iso-surface. To be more accurate, I want to use volume integral panel, and do the same averaging over the complete drop volume. The problem is, there is no way to select particulary the volume of my drop (patched as volume frac.=1 of liquid phase), so that I can calculate an average over the all cells in my drop. I can only select the complete fluid zone in my domain. Does anyone have an idea how to fix this?

Now go to Grid --> Separate --> Cells and select 'Isovalue' in the 'Registers' panel and click the right fluid zone in the 'Zones' panel. Click on separate, this will now form a new cell zone that will contain your drop.

Now go back to your Volume integral panel, you can select this newly formed cell zone to perform your calculations.